Flaviviruses have evolved principally by mutational change. Human disease caused by flaviviruses is classified as either (i) central nervous system (CNS) infection, (ii) hemorrhagic fever, or (iii) fever-arthralgia with or without rash. St. Louis encephalitis (SLE) virus is a member of the Japanese encephalitis (JE) antigenic complex, and antigenic cross-reactivities between SLE virus and other members of the complex are demonstrable with polyclonal and monoclonal antibodies. JE virus is the prototype of an antigenic complex that includes SLE and Murray Valley encephalitis (MVE) viruses, West Nile virus (WNV), and several other flaviviruses of lesser medical importance. The host range of MVE virus and susceptibility of cell cultures are provided in this chapter. Antigenic analyses differentiate strains of WNV from Africa, Europe, and the Middle East from strains isolated in India and the Far East. WNV is one of the most widely distributed of all arboviruses. Dengue virus fever is caused by four antigenically and genetically related but distinct viruses (dengue virus types 1 to 4) are distinguished by neutralization tests. Dengue hemorrhagic fever (DHF)/dengue shock syndrome (DSS) is described in detail. The chapter also talks about the techniques for virus isolation and reverse transcriptase PCR (RT-PCR) from blood of acutely ill patients, and discusses the control of Aedes aegypti for dengue virus.

Transmission cycle of SLE virus and WNV. Both viruses are transmitted in a cycle between birds and Culex mosquitoes. C. pipiens is an important vector in the northern United States and Canada. Culex quinquefasciatus is important in the southern United States, whereas C. tarsalis is important in the western United States. In addition, C. nigripalpus is a vector in Florida. Other mosquito species may be “bridge” vectors to horses, humans, and other dead-end hosts, which typically do not develop high-level viremia and do not participate in the transmission cycle. However, bridge vectors likely have a minor role compared to enzootic vectors in viral transmission to dead-end hosts.

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FIGURE 2

Transmission cycle of SLE virus and WNV. Both viruses are transmitted in a cycle between birds and Culex mosquitoes. C. pipiens is an important vector in the northern United States and Canada. Culex quinquefasciatus is important in the southern United States, whereas C. tarsalis is important in the western United States. In addition, C. nigripalpus is a vector in Florida. Other mosquito species may be “bridge” vectors to horses, humans, and other dead-end hosts, which typically do not develop high-level viremia and do not participate in the transmission cycle. However, bridge vectors likely have a minor role compared to enzootic vectors in viral transmission to dead-end hosts.

Expansion of WNV in North America, Central America, and the Caribbean, 1999 to 2006. Areas demarcated by solid lines are confirmed by virologic means in mosquitoes or vertebrates. Areas demarcated by dashed lines are those with serologic evidence in vertebrates only. Serologic evidence of WNV was found in Colombia and Venezuela in 2004, and WNV caused an equine outbreak in Argentina in 2006.

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FIGURE 6

Expansion of WNV in North America, Central America, and the Caribbean, 1999 to 2006. Areas demarcated by solid lines are confirmed by virologic means in mosquitoes or vertebrates. Areas demarcated by dashed lines are those with serologic evidence in vertebrates only. Serologic evidence of WNV was found in Colombia and Venezuela in 2004, and WNV caused an equine outbreak in Argentina in 2006.

Simplified transmission cycle of dengue and yellow fever viruses. Both viruses have a jungle cycle involving tree hole mosquitoes. In the Americas, Haemagogus transmits yellow fever, but no jungle cycle for dengue fever has been discovered. In Africa, both viruses are transmitted in jungle cycles involving Aedes spp. Yellow fever does not exist in Asia, but in Malaysia, Sri Lanka, and probably elsewhere, dengue virus is transmitted in a jungle cycle involving Aedes spp. The relationship between the jungle cycle of dengue fever and human infections is not clear. However, the jungle cycle of yellow fever is a source of human infections in the Americas and Africa. Dengue fever transmission occurs principally in urban environments, where domestic A. aegypti serves as the vector and humans serve as the viremic hosts. A similar cycle of yellow fever transmission occurs commonly in West Africa. No urban outbreaks of yellow fever have occurred in the Americas since 1964, except possibly for a very limited outbreak in Bolivia in 1997 to 1998.

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FIGURE 10

Simplified transmission cycle of dengue and yellow fever viruses. Both viruses have a jungle cycle involving tree hole mosquitoes. In the Americas, Haemagogus transmits yellow fever, but no jungle cycle for dengue fever has been discovered. In Africa, both viruses are transmitted in jungle cycles involving Aedes spp. Yellow fever does not exist in Asia, but in Malaysia, Sri Lanka, and probably elsewhere, dengue virus is transmitted in a jungle cycle involving Aedes spp. The relationship between the jungle cycle of dengue fever and human infections is not clear. However, the jungle cycle of yellow fever is a source of human infections in the Americas and Africa. Dengue fever transmission occurs principally in urban environments, where domestic A. aegypti serves as the vector and humans serve as the viremic hosts. A similar cycle of yellow fever transmission occurs commonly in West Africa. No urban outbreaks of yellow fever have occurred in the Americas since 1964, except possibly for a very limited outbreak in Bolivia in 1997 to 1998.

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